The combination of two or more polymer types into bitumen is considered desirable from the standpoint of obtaining improvements in properties that may not be possible with a single modifier. By providing a blend of the two or more types of polymers, it is conceivable that improvement in properties can be achieved over a greater range of expected pavement-service conditions.
While this goal is highly desirable, heretofore it has been difficult to achieve this result because of the inherent incompatibility of bitumen and the polymeric materials to be incorporated therein. In addition, different polymers, which, when used independently, are compatible with the bitumen, in combination, interfere with the compatibility of each other and/or the final system.
Various procedures have been attempted to improve the compatibility of bitumen and polymers, for example, by the use of low molecular weight copolymers, process oils, and by the use of specific processing conditions to promote chemical reactions, such as cross-linking.
In International patent application no. WO 93/07219 (corresponding to U.S. Ser. No. 863,734 filed Apr. 6, 1992), assigned to The University of Toronto Innovations Foundation, there is described the stabilization of dispersed polyethylene and other olefin polymers in bitumen for imparting improved properties to such asphalt. The polyethylene is maintained in a dispersed state by using a steric stabilizer which is anchored to the dispersed particles and which is compatible with the liquid medium. Such an arrangement maintains the dispersed polyethylene particles spaced from each other in the bitumen, and inhibits separation of the particulate phase from the liquid phase by the progressive coalescence of dispersed particles.
In particular, a stable dispersion of polyolefin particles in bitumen may be formed by employing a functionalized diene rubber which is covalently bonded to a functionalized polyethylene, with or without liquid polybutadiene. Cross-linking of the unsaturated structure in the functionalized butadiene molecular chain to the bitumen forms an extended polybutadiene-based network with a partially cross-linked structure anchored to each of the polymer particles by compatibility of the functionalized polyethylene with the polymer particles and swollen by the bitumen phase, to provide a gel envelope about the polymer particles, which prevents coalescence of the polymer particles.